Doppler techniques generally rely on a direct determination of the frequency shift of the primary wave caused by reflection of the wave off of the object upon which it impinges. Depending on the application, the Doppler system may utilize various sources of radiation such as, for example, acoustic, ultrasound (US), or electromagnetic. In imaging applications, for example, a Doppler system may be incorporated in a real-time scanning imaging system, such as an US imaging system.
Ultrasonic Imaging.
Various ways are known in which ultrasound can be used to produce images of objects. In the so-called “transmission mode”, for example, an ultrasound transmitter may be placed on one side of an object so as to have sound transmitted through the object to the ultrasound receiver that is placed on the other side of the object. With transmission mode methods, an image may be produced in which brightness of each pixel of an image is a function of the amplitude of the ultrasound wave that reaches the receiver (“attenuation” mode), or in which the brightness of each pixel of the displayed image is a function of the time required for the sound to reach the receiver (“time-of-flight” or “speed of sound” mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound reflected from the object back to the receiver (this is referred to as “refraction”, “backscatter” or “echo” mode).
Several backscatter methods for acquiring ultrasound data are known. In the so-called “A-scan” method, an ultrasound pulse is directed into the object by the transducer and the amplitude of the reflected sound is recorded over a period of time. The amplitude of a response signal (also referred to as an echo signal) is proportional to the scattering strength of reflecting elements in the object and the time delay is proportional to a distance separating these reflectors from the transducer. In the so-called “B-scan” method, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting response signals are recorded in a fashion similar to that of the A-scan method, and the amplitudes of these response are used to modulate the brightness of pixels on a display. The location of the transducer and the time delay values of the received response signals determine which display pixels are to be illuminated. With the B-scan method, enough data are acquired from which a two-dimensional (2D) image of the reflecting elements can be reconstructed. Rather than physically moving the transducer over the subject to perform a scan, sometimes an array of transducer elements is employed while an ultrasonic beam is electronically moved or scanned over a region of interest.
Ultrasonic transducers for medical applications are known to include one or more piezoelectric elements sandwiched between a pair of electrodes. A typical piezoelectric element is constructed of lead zirconate titanate (PZT), polyvinylidene diflouride (PVDF), or PZT ceramic/polymer composite. The electrodes of the piezo-element are connected to a voltage source, and application of voltage to the piezo-element triggers its change of dimensions at a frequency corresponding to that of the applied voltage. When a voltage pulse is applied, the piezoelectric element emits an ultrasonic wave, into the media to which it is coupled, at frequencies present in the excitation pulse. Conversely, when an ultrasonic wave strikes the piezoelectric element, the element produces a corresponding voltage across its electrodes. Typically, the front of the element is covered with an acoustic matching layer that improves acoustical coupling with the media in which the ultrasonic waves propagate. In addition, a backing material may be disposed to the rear of the piezoelectric element to absorb ultrasonic waves that emerge from the back side of the element so that they do not interfere. A number of such ultrasonic transducer constructions have been disclosed (see, for example, U.S. Pat. Nos. 4,217,684; 4,425,525; 4,441,503; 4,470,305 and 4,569,231).
When used for ultrasound imaging, the transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages (apodizing). By controlling the time delay (or phase) and amplitude of the applied voltages, the ultrasonic waves generated, in the transmission mode, by such a phase array of piezoelectric elements combine to produce a net ultrasonic wave focused at a selected point. By controlling the time delay and amplitude of the applied voltages, this focal point can be moved in a plane to scan the subject.
The same principles apply when the transducer is employed to receive the reflected sound (in the receiving mode). Specifically, the voltages produced at the transducer elements in a phase-array are summed together such that the net signal is indicative of the sound reflected from a single focal point in the subject. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the response signal received by each transducer array element.
US Doppler System.
The ultrasound (US) investigation represents a non-invasive, diffuse, and low-cost method capable of evaluating the blood velocity by exploiting the Doppler effect. The US Doppler systems employ an ultrasonic beam to measure the velocity of moving reflectors, such as flowing blood cells. The blood velocity is detected by measuring the Doppler shifts in frequency imparted to ultrasound by reflection from moving red blood cells. Accuracy in detecting the Doppler shift at a particular point in the bloodstream depends on defining a small sample volume at the required location and then processing the responses to extract the Doppler shifted frequencies. The possibility of accurately measuring the velocity of an object or the flow of liquid such as, for example, the velocity of blood flowing in human blood-vessels represents a significant opportunity for hemodynamic research and early diagnosis of cardiovascular diseases.
The real-time scanning US system provides electronic steering and focusing of a single acoustic beam and enables small volumes to be illuminated anywhere in the field of view of the instrument, and to visually identify their locations on a two-dimensional B-scan image. A Fourier transform processor faithfully computes the Doppler spectrum backscattered from the sampled volumes, and by averaging the spectral components the mean frequency shift can be obtained. Typically the calculated blood velocity is used to color code pixels in the B-scan image.
The velocity information is obtained through processing the characteristics of the Doppler-shifted US-beam. According to the Doppler effect, an object moving with velocity v and impinged upon by a planar US-wave of frequency f0 generates a response characterized by an angle-dependent frequency shift:fs=f0v cos φ/c  (Eq. 1);where φ is the Doppler angle defined between the object velocity vector and the k-vector of the wave incident upon the moving object, and c is the speed of a wave propagation in corresponding medium. One recognized limitation of this technique is that, by measuring the frequency fs, only the axial component v cos φ of the velocity can be estimated, and the knowledge of the Doppler angle φ is required to proceed with the quantitative measurement of v.
The described limitation led to a development of new techniques for estimating the velocity components along all of the spatial axes, such as techniques using dual transmitters, multiple lines of sights, multiple receivers, multi-plane detection, and speckle tracking. In most commercial US systems the problem is partially overcome by rough assessment of the Doppler angle with the help of a B-mode image and the use of the M-line and the reference line. In another example, in order to resolve the Doppler angle ambiguity, the related art proposed an approach involving the use of dual transmitters. This approach takes advantage of employing two Doppler sources such as US transducers, one of which is oriented transversely to the investigated vessel; of sensitivity of the resulting measurement to small deviations from the ideal 90° orientation of one of the Doppler sources; and of the fact that the resulting Doppler spectra are substantially symmetrical around the zero mean frequency.
A skilled artisan should recognize that currently-existing Doppler methods of measuring the velocity of a moving object endure specific shortcomings affecting either the precision of the resulting measurement, depending on a specific application, or the complexity of electronics used to carry out such measurements. For example, methods of the related art are based on the measurement of a Doppler-shift of a very high US frequency (i.e. a determination of a rather small value relative to the US frequency itself). This conventionally requires the use of a frequency demodulator and restrains the range of measurements to those of relatively high-speed objects. The use of Doppler technique in cardiovascular applications has an additional limitation imposed by insufficient spatial resolution of the Doppler technique. In particular, while the measurements of the superficial blood vessels of the body can be performed with a Doppler system operating in a continuous regime, the measurements of deep-lying blood vessels of the body often dictate that the Doppler system be operated in a pulsed regime, without which a satisfactory spatial resolution cannot be achieved. A pulsed Doppler system, however, is afflicted with a range-velocity product maximum that makes it substantially impossible for the system to measure the fastest blood flows occurring deep in the heart. The pulsed Doppler system may additionally have lowered signal-to-noise ratio. Finally, most of the Doppler systems employ transducers with such radiation patterns that limit the spatial resolution achievable during the measurement of the moving object.
Therefore, it would be desirable to have a system and method that is capable of determining the velocity of a moving object using US without being limited by the drawbacks of the prior-art.